During recent years, it has become more and more common for pipes for the transmission of fluids to be made from plastic materials.
Non-metal materials are made into various types of pipes by several methods. For instance, it is known to extrude PVC into various shaped pipes. Another more economical method is the corrugated pipe technique, which uses blow mold and vacuum mold techniques to produce a pipe which is formed as extruded tubes of a polymer such as HDPE (high density polyethylene). Systems for fabricating such corrugated pipes are sold by Corma, Inc. of Concord, Ontario. A system of this type is also described in U.S. Pat. No. 5,296,188, which is herein incorporated by reference.
Polymer material pipes are generally manufactured as continuous extrusions which are then cut into sections. Frequently, each section has a male end and a female end, and these sections are linked sequentially across the distance needed for the pipe to extend. The female end of the pipe, sometimes called a bell, receives the male end, sometimes called a spigot, of the next pipe therein.
One problem presented in such pipes derives from the fact that, during fabrication, the material from which the pipe is made is heated, and then cooled. As different portions of the pipe cool and shrink at different rates, stresses are created. If the pipe is then exposed to heat, even such as that of strong sunlight, it may cause enough warming to soften portions of the pipe.
The result of this is that portions of the bell are altered from the desired shape. As a result, the bell, which is optimally an open flared cone or skirt with a circular mouth, "flowers", i.e., deforms into a roughly clover-leaf or flower cross-section. This flowered structure presents problems when the pipe sections are to be assembled, since the puckering of this shape may make the female end of the pipe too small to easily accommodate the male end of the next pipe section. This means that the construction crew must try to pry the flowered bell apart to insert the male portion. This is inefficient in terms of labor costs and time. Moreover, the flowering can render the pipe section completely unusable.
Similarly, stacking of the pipe section can create loads in the female end of the pipe section. If environmental factors such as heat created by sunlight or high ambient temperatures are present, this can soften the material to the degree that deformation occurs.